1. Field of the Invention
The present invention relates to a display, and more particularly, to an organic electroluminescent (EL) device.
2. Background of the Related Art
Organic EL devices, also called organic light emitting diodes (LEDs), are becoming very popular because of their possible application to flat panel displays (FPDs). Organic EL devices are extremely thin, matrix-addressable and operable at a relatively low voltage, typically less than 15 volts. Furthermore, they have additional features suitable for next generation FPDs such as, among other things, little dependence on viewing angle and good device-formability on flexible substrates. Organic LEDs differ fundamentally from conventional inorganic LEDs. For Example, the charge transfer in inorganic LEDs is band-like in nature and the electron-hole recombination 15 results in the interband emission of light; while organic films are generally characterized by low-mobility activated hopping transport, and the emission is excitonic. Furthermore, organic EL devices are substantially different from conventional inorganic EL devices in other respects, notably in that in that organic EL devices are operable at low DC voltages.
Referring to FIG. 1, a typical organic EL device is shown with a first electrode 2 formed on a transparent substrate 1, a hole injecting layer (HIL) 3 and a hole transporting layer (HTL) 4 formed on the first electrode 2, a luminescent layer 5 formed on the HTL 4, an electron transporting layer (ETL) 6 and an electron injecting layer (EIL) 7 formed on the luminescent layer 5, and a second electrode 8 formed on the EIL 7. Any one or more of HIL 3, HTL 4, ETL 6 and EIL 7 may be omitted, depending on the particular device structure adopted.
Electrons and holes injected into the luminescent layer through the second electrode 8 and the first electrode 2, respectively, recombine to decay radiatively. For most organic EL devices, the charge injection barrier is higher for electrons than for holes. It is well known that the electron injection barrier may be lowered by employing a low work function material for the second electrode 8. However, low work function materials are chemically reactive, which makes it difficult to use such materials for electrodes. Accordingly, such materials are often used as a second electrode after being alloyed with one of more stable materials, as seen in the examples of Mg:Ag and Al:Li. However, such alloyed second electrodes are still less stable, more costly to form, and more difficult to deposit in a uniform film as compared to aluminum.
An even more serious problem often encountered with an alloyed second electrode of Mg:Ag or Al:Li is the frequent occurrence of cross talk or current leakage between pixels, which may be attributed to the diffusion of Mg or Li ions across organic layers of the device. This problem can be greatly alleviated if one selects aluminum as a second electrode material. However, in the case of aluminum there is a need to improve its poor electron injecting capability. The electron injecting capability of a high work function second electrode, such as aluminum, can be significantly enhanced by inserting a very thin layer (typically 0.3 nm .about.1.0 nm) of an electrically insulating material such as LiF, MgF.sub.2 or Li.sub.2 O, inserted either between an aluminum electrode and the luminescent layer, or between the aluminum electrode and the ETL(see, for example, IEEE Transactions on Electronic Devices, Vol. 44, No. 8, p 1245-1248(1997), the contents of which are incorporated herein in their entirety).
Li.sub.2 O is a particularly interesting material, in this regard, in that it is an electrically insulating material with a very low work function. The work function of alkali metals themselves is very low, and it becomes even lower when oxidized: for example, work function decreases from 2.1 eV for Cs to about 1 eV for Cs.sub.2 O. Various alkali metal compounds have reportedly been used to form an insulating buffer layer for the purpose of lowering the electron injecting barrier: e.g., Li.sub.2 O, LiBO.sub.2, NaCl, KCl, K.sub.2 SiO.sub.3, RbCl, and Cs.sub.2 O to name a few. Despite this improvement, the introduction of the insulating buffer layer poses a challenging new problem, namely, deterioration of adhesion between an EL multilayer and aluminum, with consequent reduction of life time of the device. Experimental results reveal evidence of poor adhesion either at the buffer layer/aluminum interface or at the EL multilayer/buffer layer interface. This situation is not unexpected, given the different characteristics of materials involved. In summary, organic EL devices of the related art have at least two basic drawbacks, namely, poor adhesion and short life time.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.